struck and unstruck plane

[rhohf@idcnet.com]

Michael,

Your experiment is very interesting.  I have tried to follow this thread,
please forgive me if I ask questions which were covered in messages I may have
missed.

>Here is the explanation I promised about the traces......

Also attached to the I beam is a transducer.  The transducer converts the
mechanical energy of the string to electrical energy.  It consists of two
thin copper plates about 0.4 cm x 8 cm.  One plate is positioned so that it
is parallel to the struck plane and the other is set perpendicular.  Both
plates are about 0.4 cm from the string.  A power supply provides a charge
to the plates and the change in the electric field is measured for the
string in motion through each plate separately and independently.  The
signal is then sent to a signal analyzer which records and processes the
information for the graphs.<

If I am visualizing the setup correctly, the plates are long and narrow,
parallel to the string in the 8 cm dimension and perpendicular to each other
in the .4 cm dimension.  Are they aligned to the same section of the string,
or do they measure different lengths?

The traces seem to record voltage in the plates.  How does this voltage change
relate to the string motion?  Does the voltage reflect the amplitude of the
string, or (which seems more likely) the velocity of the wire as it
oscillates?  This may seem like hair splitting, but, if it is velocity that
changes the plate voltage, then the peaks of the tracing would occur as the
wire is passing the centerline, or the rest point, of the travel.  Is it
possible to infer the amplitude from the velocity?

Your string was 78 cm long or roughly 10x the length of the plates.
Considering the complexity of string motion, is it possible that the string in
front of the plates might have sections that are moving up and down at the
same time?  How would the transducer register this case?  If the effect is to
average the motion over the 8 cm being measured, then it seems possible that a
vibrating string might register 0 volts at times on the tracing.

Bob Hohf
Wisconsin